Networx: Pouring a concrete slab

Laura Firszt More Content Now

Thursday

Jun 7, 2018 at 8:44 AMJun 7, 2018 at 8:44 AM

Concrete is one of the most user-friendly building materials around. It’s durable and inexpensive, yet can be made amazingly attractive with today’s new concrete finishes. Learn how to pour a concrete slab, the basis for many outdoor hardscape projects.

Calculate thickness and reinforcement

The purpose for which you’re pouring a concrete slab will determine the thickness of your concrete and the type of reinforcement you’ll need to install.

Parking. Concrete for additional parking, whether for vehicles used every day or just occasionally (think RV or boat), should be at least 6 inches thick and reinforced with rebar.

Garage. Similar to parking, concrete garage flooring needs to be thick and strong enough to take the weight of one or more vehicles.

Patio or pool deck. Here, pouring a concrete slab 4 inches in thickness is fine. While you may choose to reinforce it with rebar, wire mesh should be sufficient.

Floor for outbuilding. A 3- to 4-inch slab will work for the floor of a storage or tool shed, workshop, garden shed, or playhouse.

Steps to pouring a concrete slab

1. Contact your local building authority. Check whether you’ll need a permit for your project. You should also find out the setbacks so you’ll know how much distance to leave between the lot line and your construction.

2. Phone a one-call number. Verify the position of any underground cables prior to excavating.

3. Excavate — and level if necessary. Slope a patio slab approximately ¼ inch per 12 feet away from your house to allow rain to run off and avoid damage to your foundation.

4. Install subbase. Add 1-1 ¼ inches of gravel for every inch of concrete you plan to install. Tamp down for a solid subbase that will resist settling. Then dampen with a garden hose; this minimizes concrete cracking due to shrinkage.

5. Build forms. Set a stake at each corner of your future slab. Attach two side form boards (2 x 12 boards cut 3 inches longer than the intended slab width) to the corner stakes. Then add two end boards, the exact length you need, meeting the side boards at precise 90 degree angles. Nail bracing to the forms at 2-foot intervals.

6. Mix concrete. Mix 1 part Portland cement, 2 parts sand, and 4 parts aggregate (usually gravel) in a cement mixer or wheelbarrow. Then add just enough water to make a workable mixture. (Too much water will weaken the concrete and cause it to crack more readily.)

7. Pour concrete. Mound the concrete into the form, with the highest peaks 2-3 inches above the formwork.

8. Distribute. Hoe the concrete to distribute it evenly in your form, with a flat top surface.

9. Screed. Level your concrete by dragging a screed board (straight 2 x 4) evenly across it. Work from the highest point down the slope. Make sure all spaces are evenly filled.

10. Float. Use a bull-float to compress the aggregate, so that smooth gravel-less concrete sits on top of your slab.

11. Broom finish. Produce an anti-skid surface by brushing the concrete with a stiff-bristled broom.

12. Cut control joints. Create control joints about every 5-6 feet to minimize cracking. These should be equal to ¼ the slab depth (for example, 1 inch deep for a 4-inch slab or 1 ½ inch for a 6-inch slab.)

13. Edge. Smooth your slab’s edges with an edging tool.

14. Cure. Let your slab cure for maximum strength and stability. Allow 24 hours before walking on new concrete, 10 days before driving a car, and 28 days before driving a heavier vehicle on it. During this period, keep the concrete moist by spraying or ponding.

Tips

— The best weather for pouring a concrete slab is not too hot or cold — 70 degrees is ideal.